Membrane interfacial localization of aromatic amino acids and membrane protein function

摘要

Biological membranes are complex assemblies of lipids and proteins that allow cellular compart-mentalization and act as the interface through which cells communicate with each other and with the external milieu. It is well known that the interiors of biological membranes are viscous, with an effective viscosity comparable to that of light oil (Edidin 2003). In addition, membranes exhibit a considerable degree of anisotropy along the axis perpendicular to the bilayer. While the center of the bilayer (hydrophobic core) is nearly isotropic, the upper portion, only a few angstroms away toward the membrane surface (membrane interface), is highly ordered (Seelig 1977; Perochon et al 1992; White and Wimley 1994; Chattopadhyay 2003). Properties such as polarity, fluidity, segmental motion, ability to form hydrogen bonds and extent of solvent penetration vary in a depth-dependent manner in the membrane. The interfacial region in membranes (see figure 1) is the most important region so far as the dynamics and function of the membrane is concerned. The membrane interface is characterized by unique motional and dielectric characteristics distinct from both the bulk aqueous phase and the more isotropic hydrocarbon-like interior of the membrane. It is a chemically heterogeneous region composed of lipid headgroup, water and portions of the acyl chain. Overall, the interfacial region of the membrane accounts for 50% of the thermal thickness of the bilayer (White and Wimley 1994).